PHOENIX — A federal judge pushed back Thursday against a contention by the Obama Justice Department that a tough new Arizona immigration law set to take effect next week would cause “irreparable harm” and intrude into federal immigration enforcement.

“Why can’t Arizona be as inhospitable as they wish to people who have entered or remained in the United States?” U.S. District Judge Susan Bolton asked in a pointed exchange with Deputy Solicitor General Edwin S. Kneedler. Her comment came during a rare federal court hearing in the Justice Department’s lawsuit against Arizona and Gov. Jan Brewer (R).

Bolton, a Democratic appointee, also questioned a core part of the Justice Department’s argument that she should declare the law unconstitutional: that it is “preempted” by federal law because immigration enforcement is an exclusive federal prerogative.